Decompression Of Steam With Valves

- Sep 26, 2019-

When heated, water molecules are constantly destroyed and bound. Heating makes bonding between molecules fragile, and eventually some molecules break at high temperatures, producing steam or dry steam. Wet vapor is formed when some water molecules release their latent heat and form tiny droplets.

Steam is used in many industrial applications. The most common applications are process heating and power generation by driving steam turbines. In addition, steam is also used for atomization, cleaning, moisturizing and humidifying. However, when using steam, some parameter control is needed in most cases, and the steam control valve naturally becomes a part of the process control engineering.

Why Steam Control Valves are Needed in Process Control

Technically, valves are devices used to control the flow of liquid or steam in any system. The main purpose of the steam control valve is to reduce the inlet steam pressure in the process. In addition to controlling pressure, the steam control valve also controls temperature.

Boilers usually work at high pressure because low pressure operation can cause water to be carried in the steam. The low specific capacity of high pressure steam is helpful to reduce the weight of pipeline. In fact, the distribution of steam becomes easier and cheaper because of the lower cost of piping and insulation materials.

Industrial processes use steam at lower pressures. The reason is that low pressure steam has high latent heat, which greatly improves energy efficiency. The steam pressure is related to the temperature, so the temperature can be automatically adjusted by controlling the steam pressure. Reducing steam pressure is also related to plant safety requirements. Steam pressure can be controlled by a steam control valve.

In equipment using steam, steam is usually produced at high pressure and provides heat for each steam user through partial decompression. Pressure relief valves are usually designed to minimize the diameter of the steam pipeline and make the steam transportation more cost-effective.

How to Reduce Steam Pressure





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A common way to reduce pressure is to reduce the size of the steam passage by throttling. For the most basic decompression, you can simply install a traditional cut-off valve in a fixed, partially open position, or insert orifice plates into the steam flow. However, any flow fluctuation will be accompanied by corresponding pressure fluctuations. To avoid this situation, a pressure relief valve (PRV) can be used to accurately control downstream pressure. These pressure relief valves can automatically adjust the opening of the valve to ensure that the pressure remains constant even if the flow rate fluctuates.

Advantages of pressure relief valves

Pressure can be maintained by using a combination of start-up control valves, pressure sensors and controllers. However, pressure relief valves have the advantage of controlling pressure by fully automatic independent operation without any external power. It can be made very quickly by instant perception and adjustment of downstream pressure. Response action

Butterfly valve

Types of steam relief valves

In pressure relief valves, the mechanism that automatically regulates downstream pressure usually utilizes the balance of force between steam pressure and regulating spring. At present, this is a common concept used by almost all pressure relief valves. But there are two different ways to achieve this mechanism to control valve opening:

Pilot-free, direct-acting pressure relief valve: The adjusting spring exerts the setting force of down-regulating pressure directly on the main valve.

Pilot relief valve: The adjusting spring directly exerts the setting force of the downward pressure on the pilot valve, which is smaller and different from the main valve.

The following is an overview of the characteristics of each type of PRV.

Direct acting pressure relief valve (without pilot)

For small loads that do not require very precise pressure control.

Advantages: Small size, low price, easy installation.

Disadvantage: A higher pressure drop (with changes in set pressure) than a pilot PRV.

In direct acting relief valves, the opening of the valve is directly dependent on the movement of the regulating spring. If the spring is compressed, it will generate an opening force on the valve, thereby increasing the flow rate.

When the pressure accumulates downstream, the downstream pressure is transferred to the bottom of the regulating spring (usually close to the bellows or diaphragms), where the upward force is balanced with the spring compression force. The spring pressure of the opening valve is limited to offset the change of downstream pressure while achieving sufficient spring sensitivity. The final result is to simply control the pressure at the high flow rate where pressure drops through the orifice plate.

Pilot-operated reducing valves

For larger loads that require very precise pressure control.

Advantages: Compared with direct-acting mode, it can accurately control pressure and respond quickly to load changes, and can be used in a larger flow range.

Disadvantage: Large size, high price.

In the pilot relief valve, the pilot valve is used to load the piston or diaphragm, thereby increasing the downward force used to open the larger main valve. This increases the flow at a lower pressure compensation (depressurization). The opening and closing of the pilot valve is controlled by the force balance between the regulating spring and the secondary pressure in the same way as the direct acting valve.

stainless steel Check valve

But in the pilot PRV, the opening and closing of the pilot valve will transfer pressure purposefully to the piston or diaphragm of the main valve. Then, the pilot flow pressure will produce a downward force, which will be amplified by the piston or diaphragm surface, so that the main valve opens more, so that the flow rate reaches a high level.

In the pilot pressure relief valve, the pilot valve is used to load the piston or diaphragm, thereby increasing the downward force applied to the larger main valve.

In the pilot relief valve, the pilot valve is used to load the piston or diaphragm, thereby increasing the downward force used to open the larger main valve.

Because the use of piston or diaphragm will magnify the downward force, the slight change of pilot valve opening will lead to greater changes in flow through the main valve and downstream pressure. Therefore, in order to achieve rapid response in a larger range of steam flow, the spring force on the control valve hardly needs to change. Compared with the direct-acting type, the main advantages of the valve are fast response and tight pressure control.


From the above characteristics, it can be seen that the function and application of the non-pilot direct acting pressure relief valve are quite different from that of the pilot type pressure relief valve. In short:

When the load is small, the direct acting pressure relief valve can be used, and the downstream pressure can be allowed to decline to a certain extent. They are usually used under light load conditions. Pilot relief valve can respond to different load conditions quickly, while maintaining stable secondary pressure where precise pressure control is required. They are usually used in the case of heavy load.

Typical applications of steam use devices:

Low-load applications, such as sterilizers, unit heaters, humidifiers and small process equipment, can usually be decompressed using a simple direct-acting PRV.

If the flow rate is large, such as the steam pipeline, the load may fluctuate greatly according to the operation status of the receiving equipment.

carbon steel Gate valve

This load change and capacity require the use of pilot relief valves to reduce pressure. In addition, the amount of steam used in some equipment may vary considerably between start-up and normal operation. Such a large change may also require the use of pilot PRV for decompression.